The present invention relates to an automatic voltage regulator for an AC generator with a field winding, namely a self-excited generator or a separately excited generator.
A conventional automatic voltage regulator for an AC generator includes a thyristor connected in series with a field winding to control the field current. The thyristor receives its gate signal from its anode through a resistor, so that it conducts a field current whenever the anode is forward biased and the output of the generator will be increased, unless some inhibiting operation, i.e. voltage regulation is effected. To accomplish voltage regulation, the voltage regulator comprises a voltage detection circuit adapted to detect the output voltage of the generator and to provide a rectified voltage with a ripple. As the instantaneous value of the rectified voltage exceeds a predetermined value, triggering of the thyristor is inhibited by shorting the gate and cathode of the thyristor by means of a transistor. Accordingly, after the output voltage of the generator has been established, application of the gate signal to the thyristor is made possible only when the instantaneous value of the rectified voltage falls below the predetermined value at the "valleys" of the rectified voltage waveform.
Where the exciting voltage is in phase with the output voltage of the generator, the thyristor is either made conductive throughout 180.degree. or made non-conductive throughout 180.degree., because the rectified voltage waveform is such that its "valleys" appear at the moments when the instantaneous value of the exciting voltage begins to rise. This is unsatisfactory because the resultant output voltage has substantial fluctuation.
Where an exciting winding for providing a field current is positioned 90.degree. (electrical angle) apart from the armature winding, the "valleys" of the rectified voltage appear when the exciting voltage is approximately at its peaks. The angle at which the instantaneous value of the rectified voltage falls below the predetermined value varies, but within the range of from 0.degree. to 90.degree., so that the conduction angle through which the thyristor is conductive cannot be smaller than 90.degree.. Consequently, if a smaller average angle of conduction corresponding to smaller field current is needed, conduction of the thyristor through 90.degree. or more and non-conduction through 180.degree. are repeated, that is irregular operation occurs, resulting in undesirable fluctuation of the generator's output.